This invention relates to a process for sulfonating liquid organic feed stocks and more particularly it relates to a sulfonation process, using sulfur trioxide (SO.sub.3) as the sulfonating agent, in which provision is made for treating the effluent gases from the sulfonation process to recover for reuse any SO.sub.3 or SO.sub.2 therein and to eliminate therefrom noxious fumes before discharging the gases to the atmosphere.
Sulfonation of a wide variety of organic liquid feedstocks, and particularly relatively highly refined hydrocarbons, such as "detergent alkylate", benzene, toluene and the like, has been practiced conventionally for many years. Although several different sulfonating agents, such as concentrated sulfuric acid, oleums of various strengths, and SO.sub.3 have been used, treatment with SO.sub.3 is usually the process of choice, since the reaction is essentially instantaneous and proceeds to completion, thus minimizing equipment requirements for a given rate of production.
In a common process for sulfonation with SO.sub.3, the SO.sub.3 in vapor form is mixed with dry air and brought into contact with the liquid feed stock to produce a sulfonic acid which is then neutralized to form sulfonate. Although the desired sulfonated product is quickly and simply obtained, the effluent gases from the sulfonation step present a disposal problem, since they contain, in addition to excess SO.sub.3 not consumed in the sulfonation step, vapors of the liquid feed stock, as well as SO.sub.2 formed during the sulfonation step and perhaps some residual SO.sub.2 introduced into the sulfonation process with the SO.sub.3. Generally, the amount of SO.sub.2 in the effluent gases is sufficiently small that it can be removed from the effluent gases by scrubbing, without economic disadvantage. For example, with some relatively highly refined hydrocarbons, the SO.sub.2 content of the effluent gas is only 2-3 wt. % of the SO.sub.3 introduced at the sulfonation step.
In the case of sulfonating crude oil or gas oil, however, the SO.sub.2 content of the effluent gas can be in the range 5-15 wt. % of the SO.sub.3 introduced at the sulfonation step. "Gas oil" is a term conventionally used in the petroleum industry to describe a hydrocarbon friction near the bottom "cut" in the atmospheric distillation of petroleum, or its equivalent.
Therefore, in the sulfonation of crude oil or gas oil, or in the sulfonation of relatively highly refined hydrocarbon at very large rates of sulfonation (e.g., 50 to 100 million pounds of sulfonate per year), the amount of SO.sub.2 (and SO.sub.3) in the effluent gases from the sulfonation step can be quite large, and this amount represents an excessive economic loss unless the SO.sub.2 (and SO.sub.3) in the effluent gases is recovered for re-use in the sulfonation step.
Recent proposals for tertiary recovery of petroleum involve the use of sulfonated petroleum fractions which are injected into wells to increase the recovery of petroleum trapped within the rock or sand formation. For such use, large quantities of sulfonated petroleum fractions, typically having a molecular weight of 300-1000, are necessary. These large quantities are produced by SO.sub.3 sulfonation at very large rates, and, in doing so, substantial quantities of SO.sub.2 are formed.
Moreover, in view of the increasingly rigorous standards relating to atmospheric pollution, a sulfonation effluent gas stream containing substantial concentrations of SO.sub.2 and SO.sub.3 cannot be discharged directly to the atmosphere, and, particularly in the case of a plant treating large volumes of feed stocks, disposal of the effluent gases from the sulfonation step presents a serious problem.